In the prior art, it is well known to make aluminum alloy can stock using ingot processing. In these prior art methods, the aluminum alloy is cast into ingot form, homogenized/heated in soaking pits or furnaces and subsequently hot rolled. The hot rolled material is then furnace annealed or self-annealed and cold rolled to can stock final gauge. Can stock derived from ingot casting is beneficial in that the homogenization/soaking pit practice used for cast ingots contributes to increased alpha phase content in the product. Higher alpha phase content is desirable since it improves use of the product in a can making operation and enhances die life by reducing pickup or coating of the ironing dies.
Ingot processing of can stock is disadvantageous for several reasons, such as, the need for an ingot break down rolling mill, the need for increased material handling operations of the ingots, need for scalping ingots resulting in metal loss, intensive energy consumption, product reworking and low yields.
Continuous casting methods have been proposed to overcome the problems associated with ingot processing of can stock. U.S. Pat. No. 5,104,465 to McAuliffee et al. discloses a method of making aluminum sheet for can stock wherein the aluminum sheet is continuously chill block cast. The alloy of the McAuliffee et al patent utilizes higher manganese and magnesium concentrations then those levels in ingot processed can stock, e.g., AA3104. According to this patent, the final cold rolled gauge material is sheared and processed into a finished aluminum can.
Making aluminum alloy can stock from continuously cast material is not without its disadvantages. Typically, earing percentage in continuous cast product is high, the high earing percentage interfering with the drawing and ironing operation of can making, which results in lower productivity and lower yield due to need for greater trimming of cans. Further, these continuously cast materials exhibit poor formability in high cold work tempers as measured by the minimal spread between ultimate tensile strength and yield strength or percent elongation. In addition, the relative percentage of the alpha phase in the can stock is much lower than that found in can stock produced by ingot processing.
Another drawback associated with the prior art is the fact that AA3104/3004 type alloys, which are commonly used for can stock, are limited due to their inherent non-heat treatable nature.
In view of the disadvantages noted above, a need has developed to provide a method for making aluminum alloy continuously cast can stock which has low earing, a high alpha phase percentage and good formability. In response to this need, the present invention provides a method for making aluminum alloy can stock using continuous casting in combination with annealing and cold rolling to final gauge followed by a heat treating step which increases the spread between ultimate tensile strength and yield strength for improved formability.